The present invention relates generally to a non-volatile memory device and a memory system including the same, and more particularly relates to a non-volatile memory device for reading data by sensing accurately threshold voltage of a selected memory cell in a read operation and a memory system including the same.
Recently, demand has increased for non-volatile memory device, which does not require a refresh function of periodically rewriting data, and which allows electrical programming and erasing of data.
The non-volatile memory device performs a read operation by sensing voltage of a bit line after applying a read voltage to selected word line and providing a pass voltage to an unselected word line. The threshold voltage of memory cells connected to the unselected word line that is adjacent to the selected word line increases due to coupling effect of the read voltage applied to the selected word line. As a result, resistance of the memory cells connected to the unselected word line increases, and the threshold voltage of the selected memory cell increases. Accordingly, erroneous data may be read in the read operation.
In addition, a multilevel cell has many states, and the highest threshold voltage value of the uppermost threshold voltage distribution of threshold voltages for determining state of a memory cell increases gradually. Accordingly, the pass voltage applied to the memory cells not selected should be increased. However, when the pass voltage increases, a read disturbance phenomenon may occur as voltage difference between a gate of the memory cell and a substrate augments in the read operation.